A magnetic random access memory (MRAM) is a solid-state non-volatile magnetic storage device. Bits of data are stored in small magneto-resistive elements. For example, in a magnetic tunnel junction (MTJ) magneto-resistive element, two ferromagnetic layers, a pinned magnetic layer and a sense magnetic layer, are separated by an insulating tunnel barrier. Magneto-resistance results from the spin-polarized tunneling of conduction electrons between the ferromagnetic layers. The tunneling current depends on the relative orientation of the magnetic moments of the two ferromagnetic layers.
The magnetization direction of the sense layer is used for information storage. The resistance is either low or high, depending on the relative magnetization direction of the sense magnetic layer with respect to that of the pinned magnetic layer. The magnetization directions are either parallel (P) or antiparallel (AP).
In conventional MRAM arrays, orthogonal lines pass under and over the magneto-resistive elements, carrying current that produces magnetic fields used to switch the individual elements in the MRAM array. The magneto-resistive elements are designed so that magnetization direction of the sense magnetic layer will not switch when current is applied to just one line, but will switch when current is applied to both lines. In practice, however, large switching distributions of the magnetic tunnel junctions in an MRAM array can result in errors. That is, some half-selected magnetic tunnel junctions switch in the presence of only a single applied current, and some selected magnetic tunnel junctions do not switch in the presence of two orthogonal applied currents.
It is a continuing goal to increase MRAM storage density. Increasing the storage density increases the amount of information that can be stored per unit area. However, increasing the storage density also can result in increased potential for errors.